Microdetonator assembly

ABSTRACT

A cylindrical enclosure, or can, is formed with one end closed and the other end open. The can is partially filled with a first layer of a secondary explosive constituting an output charge, and a second layer of a primary explosive constituting a transfer charge. An explosive header is inserted into the can adjacent to the transfer charge, and the open end of the can is crimped inwardly to secure the header. The header is assembled with two concentric electrodes which are gold plated at a header face. A heater wire is positioned across the face, and is bonded to the electrodes by a first probe which presses against the wire and passes sufficient electric current to heat and soften the gold plating about the wire. Excess wire is severed by a second probe which passes sufficient electric current to melt the wire.

United States Patent Farrand et al.

[ Aug. 29, 1972 [54] MICRODETONATOR ASSEMBLY [72] Inventors: Weston B. Farrand, Palo Alto; John N. Gardner, Aptos; Robert M. Valenti, Sunnyvale, all of Calif. [73] Assignee: Space Ordinance Systems Inc., Saugus, Calif.

[22] Filed: Oct. 19, 1967 [21] Appl. No.: 676,557

[52] US. Cl ..86/1, 102/27 [51] Int. Cl. ..F42c 19/12 [58] Field of Search ..86/1; 102/28 EB, 70.2 1

[56] References Cited UNITED STATES PATENTS 2,687,667 8/1954 Gunther ..102/28 X 2,880,671 4/1959 Lutz et a1 ..102/28 2,964,835 12/1960 Hay ..102/28 X 3,291,046 12/1966 Dahl ..102/28 3,340,807 9/1967 Burr et al ..102/28 EB 2,515,780 7/ 1950 Lefren ..86/1 2,873,642 2/ 1959 Weingrad et al. ..86/1

2,942,513 6/ 1960 Seavey et al ..86/ 1 2,965,033 12/ 1960 Home et al. .....86/l X 3,351,012 11/1967 Wilson ..102/28 EB Primary Examiner-Verlin R. Pendegrass Attorney-Andrew G. Pullas [57] ABSTRACT A cylindrical enclosure, or can, is formed with one end closed and the other end open. The can is partially filled with a first layer of a secondary explosive gold plating about the wire. Excess wire is severed by i a second probe which passes sufficient electric current to melt the wire.

7Claims,5DrawingFigures P'ATENTEDws 29 1912 v r l5 I N VE N TORS WESTON B. FARRAND JOHN GARDNER ROBERT M. VALENTI ATTORNEY MICRODETONATOR ASSEMBLY This invention relates to electric detonators, and more particularly, to methods and means for fabrication of miniature devices for electrically detonating explosive charge.

Various types of explosives are known and used in ordnance and in construction. These explosives may be generally classified into primary and secondary types. Primary explosives may be easily detonated by the application of heat or shock, but primary explosives are of low power and not particularly useful in themselves. Secondary explosives are more powerful, but are more stable and not easily detonated. A detonation device may include a primary explosive to initiate the detonation and a further secondary explosive to produce the sufficient power and shock devices necessary for the detonation of a larger and more powerful explosive mass.

It is an object of this invention to provide an improved detonation device which may be fabricated on a miniature scale; and more particularly, it is an object to provide an improved method for fabrication of miniature detonators.

A further object of this invention is to provide a detonator which may be assembled in two basic parts including a metal enclosure or can with explosive therein and a header which may be pre-assembled and tested, and then inserted as a closure into the metal can containing the explosive.

A further object of this invention is to provide a method and means for bonding a heater wire across the electrical terminals of a header, and more particularly, it is an object to provide a technique for bonding miniature heater wire upon electrodes of the header, and for severing and trimming the excess wire therefrom.

Numerous other objects and advantages will be apparent throughout the progress of the specification which follows. The accompanying drawing illustrates a certain exemplary embodiment of this invention and the views therein are as follows:

FIG. 1 is an enlarged section of the detonator of this invention;

FIG. 2 is a section along the plane 2-2 of FIG. 1 showing the face of the header with a heater wire in place thereon;

FIG. 3 is a section illustrating a step in the fabrication of the detonator wherein a separately assembled header is inserted into the metallic enclosure containing explosives;

FIG. 4 is a section illustrating a method for bonding a fine heater wire to the electrodes of a header, and for severing the excess wire therefrom; and

FIG. 5 is a further enlarged section illustrating the method for bonding the heater wire to an electrode.

Briefly stated, according to a preferred form of this invention, the detonator comprises a metal enclosure or can 1 1 which is closed at one end 12 and open at the other end. The enclosure is partially filled with a secondary explosive which constitutes an output charge 13, and thence a layer of primary explosive is inserted into the can 11 to provide a transfer charge 14. A pre-assembled header 15 is inserted into the open end of the can 11 against the transfer charge 14, and is secured therein by crimping the open end of the can inwardly at 16. The pre-assembled header 15 includes two concentric electrodes 17 and 18 with a non-conductive material such as glass or plastic 19 interposed therebetween. Both electrodes 17 and 18 terminate at a face 21 of the header l5, and the terminal ends of the electrodes 17 and 18 are plated with a soft metal such as gold. A bridge wire or heater wire 22 is bonded to the electrodes 17 and 18 by a probe 23 which presses the wire 22 into the plating material 24, and passes sufficient electric current through the junction between the wire 22 and the electrode 17 to soften the plating material and efiect a bond or weld 25 therebetween. After the bonding operation, another probe 26 is used to trim away the excess wire. The probe is brought into contact with the wire and sufficient electric current is passed therethrough to melt and sever the wire for removal of the excess.

As indicated above and as shown in FIG. 1, the miniature detonator of this invention comprises a charge 13 of a secondary explosive, a transfer charge 14 of a primary explosive and a header 15 all enclosed in a metal can 11. As shown in FIG. 3 the can may be placed in an upright position for filling with explosive. The can 11 may be approximately half filled with secondary explosive which is tamped or compressed to form the output charge 13. Thence the remaining space within the can 11 is partially filled with the primary explosive which is tamped or otherwise compressed to form the transfer charge 14 which is in direct contact with the primary explosive. A pre-assembled header 15 is thence inserted into the can and the open end of the can is crimped inwardly 16 to secure the header and the layers of explosive therein. In a final operation the can 1 1 with the header 15 therein is hermetically sealed with a high melt wax 27 to exclude moisture from the interior of the assembled detonator.

A detonator may be embedded in further high explosive such that the output charge 13 lies closely adjacent to the further high explosive (not shown). On the other hand, the assembly of the whole explosive device may be imperfect such that a gap will exist between the output charge and the further explosive to be detonated. The detonator of this invention is so arranged that the detonation of the output charge 13 will separate the rearward end 12 of the can 11 of the assembly, and this rearward end 12 will be propelled as a projectile toward the further explosive to be detonated.

As shown in FIG. 3 the can 11 is formed with one end closed and the other end open. The closed end 12 includes a dimple or concave section 20. Thereafter the output charge 13 is inserted into the can 11 and will likewise be formed with a concave surface or dimple, since the output charge will assume the shape of the can. Because of the concave or hollowed shape of the output charge and can end 12 the velocity of the can end resulting upon detonation will be enhanced by a plate shaped charge effect, to provide increased shock from the can end of detonation device to a layer or mass of high explosive material (not shown) which will ultimately be assembled with the detonator device of this invention. Detonators have been constructed and tested in accordance with this invention, and it has been found feasible to detonate a secondary explosive across a gapwhich may be as great as one-fourth inch from the concave end of the detonator.

The header 15 is assembled with an inner electrode 17 and an outer electrode 18 and with an electrically non-conductive material such as glass or plastic 19 therebetween. As indicated heretofore, the concentric electrodes and the dielectric material 19 as formed with a flat terminal surface 21. In an exemplary embodiment of this invention which has been made and tested, glass was used as the dielectric material 19 and a stainless steel known as Kovar was used as the concentric electrodes 17 and 18. Kovar is an electrically conductive metallic alloy having substantially the same thermal coefficient of expansion as glass, such that the two electrodes may be assembled and held in place while molten glass is poured there between; and the assembly may be cooled without cracking the glass which adheres to the Kovar.

A further step in the assembly of the header 15 is the plating of the electrodes. In the exemplary model, the plating step was accomplished by tumble plating of gold. The gold will adhere readily to the Kovar, but will not adhere to the glass; and therefore, the entire assembly may be subjected to the tumble plating, but only the metallic electrodes will receive a plating. That gold settling upon the glass surfaces may be easily wiped or washed away, or cleaned with an air jet without damage to the plated metallic surfaces. Obviously, the outer cylindrical surface of the electrode 18 will likewise be plated in this process, but this is not detrimental to the operation of the assembly, indeed, the plated outer surface will assure a good electric contact between the electrode 18 and the outer can 11 of the detonator.

After the electrodes 17 and 18 have been plated with gold or other suitable electrically conductive material, a bridge wire 22 is bonded between the electrodes 17 and 18. This step is accomplished by aligning a plurality of headers 15 within a metallic holder or conveyor 29. With the headers 15 so aligned, a single strand of fine heater wire 31 is stretched in alignment across the entire row of headers 15. A probe 23 is then brought into contact with the wire 31 to effect a bond 25 at each of the electrodes. The probe 23 presses the heater wire 22 downwardly into the soft gold plating 24 while an electric current is passed therethrough for partially melting and softening the gold at the junction. As shown in FIG. 4 an electric power source, symbolized by a battery 32, and a ground connection 33, causes a flow of electric current through the probe 23, the wire 22 and the electrode 17. After the bonds have been effected with the probe 23, a cutting or severing probe 26 is used to trim away the excess wire 31. The cutting probe 26 is more pointed to concentrate a flow of electric current therefrom. A power source, symbolized by a battery 34 and a ground connection 35 causes electric current to flow from the probe 26 to heat and to melt the wire 31 for removing excess segments therefrom.

As shown in FIG. 4 a bridge or heater wire 22 has been welded between the electrodes of the first (left hand) heater 15. A test circuit 36 is thence electrically connected between the inner conductor 17 and a ground connection to the metallic tray or holder 29, and is used to check the electrical resistance value between the electrodes 17 and 18 of the header 15. In the exemplary model of this invention a properly welded bridge wire 22 would provide an electric resistance in a range of 0.l to ohms. The electrical resistance check provides an indication for quality control to assure a satisfactory bond of the fine bridge wire 22 to both of the electrodes.

In the exemplary embodiment of this invention, the over-all length of the detonator, exclusive of the extending electrode 17, was approximately one-fourth inch and a diameter of 0.100 inch. Smaller detonators having diameters of the order of 0.050 inch are feasible. The inner electrode of the exemplary model has a Kovar wire having a diameter of 0.0185 inch. The outer electrode 18 was of a thickness of the order of 0.01 inch, and the bridge wire or heater wire was of tungsten which was drawn to 0.00015 inch. The techniques heretofore described have proved successful in effecting a bond of the fine tungsten wire to the much larger Kovar electrodes.

After the bridge wire 22 has been bonded in place, a coating of primary explosive 37 is applied to the face 21 of the header 15. The primary explosive coating consists of lead styphnate mixed with an ethyl cellulose binder. This is initially applied as a slurry such that the lead styphnate and ethyl cellulose is mixed with butyl acetate. The slurry is applied to the face 21 of the header embedding the bridge wire 22 therein. Immediately after application of this slurry all of the excess slurry material will be removed from the pheripheral parts of the header such that only the face 22 receives the coating. This method of handling an explosive is not hazardous since the lead styphnate in slurry form is very insensitive. All excess explosive material may be removed from the sides and other surfaces while the slurry is wet. The coating 37 is allowed to dry upon the face surface 21 to provide the complete assembly of the header 15 which may thence be placed within the can 1 1 of the detonator as described above.

In an exemplary embodiment of this invention, the explosive train comprised first the thin layer or coating of lead styphnate 37 which constituted a prime load. Secondly, the transfer charge was of lead azide, and the output charge of cyclotetramethylenetetranitramine more commonly called HMX. Alternatively, the output charge may be cyclotrimethylenetrinitramine more commonly called RDX. Lead styphnate is a primary explosive which may be detonated by a hot wire. Therefore, the detonation may be accomplished by passing sufficient electric current through the connecting electrodes 17 and 18 to heat the wire 22. The electrode 18 is electrically grounded to the can 11, and therefore, the external electric circuit (not shown) will be connected between the wire electrode 17 and the grounded can 1 1. When the circuit is complete the wire 22 will be heated to detonate the prime load 37. Upon detonation of the prime load 37, the transfer charge 14 will detonate thereby causing detonation of the output charge 13. Detonation of the charge 13 will create a high velocity plate from the concave end thereof to detonate a larger mass of explosive which may be placed in spaced relation to the detonator.

Detonators have been built in accordance with this invention which can be set with less than 1000 ergs of electrical energy passed through the heater wire 22. This energy is obtainable from a discharge of a capacitor which may be charged by a battery.

The invention is claimed as follows:

1. A method for fabricating a detonator comprising assembling an explosive header formed of a pair of concentric electrodes terminating in a common face of the header and a heater wire positioned across the face of the header, said heater wire being bonded to the electrodes by pressure and an electric current, said heater wire being severed at the electrodes by a probe making contact therewith and passing electric current therethrough to melt the wire, forming a cylindrical enclosure of metal with one end closed and the other end open, partially filling the cylindrical enclosure with a secondary explosive selected from the group consisting of cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine to form an output charge at the closed end of the enclosure with space remaining at the opened end of the enclosure, partially filling the remaining space with a primary explosive to form a transfer charge centrally disposed within the enclosure, and inserting the explosive header into the enclosure adjacent to the transfer charge.

2. A method for fabricating a detonator in accordance with claim 1 wherein the face of the header is plated with a soft low melting point metal which adheres only to the electrodes, said heater wire being bonded to the plated electrodes by a probe which exerts pressure thereagainst and passes an electric current therethrough to fuse the plating material upon the heater wire.

3. A method for fabricating a detonator in accordance with claim 2 wherein the plating material is gold.

4. A method for fabricating a detonator comprising assembling an explosive header, forming a cylindrical enclosure of metal with one end closed and the other end opened, partially filling the cylindrical enclosure with a secondary explosive to form an output charge at the closed end of the enclosure with a space remaining at the opened end of the enclosure, partially filling the remaining space with a primary explosive to form a transfer charge centrally disposed within the enclosure, and inserting the explosive header into the enclosure adjacent to the transfer charge, said explosive header including a pair of concentric electrodes terminating in a common face of the header, a heater wire being positioned across the face of the header, said heater wire being bonded to the electrodes by pressure and an electric current, said heater wire being severed at the electrodes by a probe making contact therewith and passing electric current therethrough to melt the wire.

5. A method for fabricating a detonator in accordance with claim 4, wherein the face of the header is plated with a soft low melting point metal which ad heres only to the electrodes, said heating wire being bonded to the plated electrodes by a probe which exerts pressure thereagainst and passes an electric current therethrough to fuse the plating material upon the heating wire.

6. A method of fabricating a header for a detonator comprising forming a pair of spaced electrodes, filling the space between said electrodes with a dielectric, and bonding by pressure and an electric current a heater wire to exposed surfaces of said electrodes across said dielectric, and severing said heater wire at the electrodes by melting the wire.

7. A method as defined in claim 6, wherein said heater wire is severed at the electrodes by a probe making contact therewith and passing electric current therethrough to melt the wixe. 

1. A method for fabricating a detonator comprising assembling an explosive header formed of a pair of concentric electrodes terminating in a common face of the header and a heater wire positioned across the face of the header, said heater wire being bonded to the electrodes by pressure and an electric current, said heater wire being severed at the electrodes by a probe making contact therewith and passing electric current therethrough to melt the wire, forming a cylindrical enclosure of metal with one end closed and the other end open, partially filling the cylindrical enclosure with a secondary explosive selected from the group consisting of cyclotetramethylenetetranitramine and cyclotrimethylenetrinitramine to form an output charge at the closed end of the enclosure with space remaining at the opened end of the enclosure, partially filling the remaining space with a primary explosive to form a transfer charge centrally disposed within the enclosure, and inserting the explosive header into the enclosure adjacent to the transfer charge.
 2. A method for fabricating a detonator in accordance with claim 1 wherein the face of the header is plated with a soft low melting point metal which adheres only to the electrodes, said heater wire being bonded to the plated electrodes by a probe which exerts pressure thereagainst and passes an electric current therethrough to fuse the plating material upon the heater wire.
 3. A method for fabricating a detonator in accordance with claim 2 wherein the plating material is gold.
 4. A method for fabricating a detonator comprising assembling an explosive header, forming a cylindrical enclosure of metal with one end closed and the other end opened, partially filling the cylindrical enclosure with a secondary explosive to form an output charge at the closed end of the enclosure with a space remaining at the opened end of the enclosure, partially filling the remaining space with a primary explosive to form a transfer charge centrally disposed within the enclosure, and inserting the explosive header into the enclosure adjacent to the transfer charge, said explosive header including a pair of concentric electrodes terminating in a common face of the header, a heater wire being positioned across the face of the header, said heater wire being bonded to the electrodes by pressure and an electric current, said heater wire being severed at the electrodes by a probe making contact therewith and passing electric current therethrough to melt the wire.
 5. A method for fabricating a detonator in accordance with claim 4, wherein the face of the header is plated with a soft low melting point metal which adheres only to the electrodes, said heating wire being bonded to the plated electrodes by a probe which exerts pressure thereagainst and passes an electric current therethrough to fuse the plating material upon the heating wire.
 6. A method of fabricating a header for a detonator comprising forming a pair of spaced electrodes, filling the space between said electrodes with a dielectric, and bonding by pressure and an electric current a heater wire to exposed surfaces of said electrodes across said dielectric, and severing said heater wire at the electrodes by melting the wire.
 7. A method as defined in claim 6, wherein said heater wire is severed at the electrodes by a probe making contact therewith and passing electric current therethrough to melt the wire. 